This is a question I have been asked on a number of occasions and I believe it could be answered in many different ways by many different people...
I have many altars, I have permanent altars that face the cardinal directions and I meditate in front of them according to my intuitive sense. I also create temporary altars wherever I go. When I travel I collect things from where I am as well as things I have brought so I have a sacred space on the road, which also reminds me of car altars, oh the things that can line the dashboard. Even in my bathroom the old built in soap holder is now the housing for a small bathroom altar.
There are as many different kinds of altars as there are people, try googling or looking on flickr really its quite fascinating! There are completely ornate high mass church altars, altars found on the sides of roads and then lovely little kitchen altars. Altars speak so much about their creators...look around perhaps you have an altar and you didn't even know it ;)
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above is production still (shot by Leslie Peters) for the peice Neverman's Land by Tristan R Whiston